A new study published in 'Science' found the ice in Greenland is melting five times faster than in the early 90s, part of what accounts for a 20 percent rise in sea level over the past two decades. NBC's Anne Thompson reports.

What had been a blurry picture about polar ice — especially how it impacts sea levels — just got a whole lot clearer as experts on Thursday published a peer-reviewed study they say puts to rest the debate over whether the poles added to, or subtracted from, sea level rise over the last two decades.

"This improved certainty allows us to say definitively that both Antarctica and Greenland have been losing ice," lead author Andrew Shepherd of the University of Leeds in Britain, told reporters. Not only that, but the pace has tripled from the 1990s, the data indicate.

Combining satellite data from dozens of earlier studies, the study "shows that the Antarctic and Greenland ice sheets have contributed just over 11 millimeters (0.4 inches) to global sea levels since 1992," he added. Two-thirds was from Greenland, a third from Antarctica.

NASA Earth Observatory

This 20-mile-long rift on Antarctica's Pine Island Glacier, seen from a satellite on Oct. 26, will eventually calve off, possibly in the next few months, creating an iceberg the size of New York City. While that won't raise sea levels since the glacial tongue sits on water, the loss could speed up the flow of ice from Antarctica's mainland into the sea.

That's 20 percent of all sea level rise over the last two decades, with the rest mostly from thermal expansion of waters due to warming sea temperatures, the authors noted. In recent years, however, the percentage "has gone up significantly" to nearly 40 percent, added co-author Michiel van den Broeke from Utrecht University in the Netherlands.

Published in the journal Science, the study was based on input from 47 experts at the 26 institutes that produced earlier studies with wild variations. Some of those studies estimated melt was raising sea levels by up to 2 millimeters a year, Shepherd noted, while a few said that overall polar ice was growing, and thus countering sea level rise.

Much of the discrepancy was due to data showing that Antarctica's vast eastern ice sheet was adding, not losing ice.

Most of that loss is in western Antarctica — at places like Pine Island Glacier, where an iceberg the size of New York City is set to calve off. The iceberg itself won't raise sea levels since that ice is already atop water, but thinning glaciers mean that ice on the mainland can make its way downhill to the sea faster.

ESA/NASA/Planetary Visions

Based on the new study in Science, this chart shows changes in global sea level due to ice sheet melting since 1992. The background image shows thickening (blue) and thinning (red) of Antarctica's ice sheets over the same period.

Even more dramatic, Ivins said, is that Greenland "is losing mass at about five times the rate today as it was in the early 1990s."

Greenland's melt rate has gone from 55 billion tons a year in the 1990s to nearly 290 billion tons a year recently, according to the study.

A top ice expert who was not a study co-author told NBC News that the new data mark "an important step forward" in better estimating future sea level rise.

"While we had a basic picture of what was going on, it was an incomplete and blurry one," said Mark Serreze, director of the National Snow and Ice Data Center at the University of Colorado-Boulder. "We needed to step back and take a fresh look, making the best use of all of the different data sources that we have.

"With this study," he added, "we now have a lot confidence in how the ice sheets are behaving."

The findings come as nations negotiate in Qatar over a new climate treaty to replace the 1997 Kyoto Protocol, which aimed to reduce carbon dioxide and other greenhouse gases tied to a warming Earth.

And while a 0.4 inch rise in sea levels over 20 years doesn't sound like much, many experts fear further warming will accelerate the polar melt. The ice sheets would raise sea levels by more than 200 feet if they completely melted over centuries — not likely, but even a tenth of that would have catastrophic impacts on coastal areas.

"It’s really critical that these measurements are sustained and several satellites are beginning to fail," noted Ian Joughin, a University of Washington researcher.

"If we really want to have meaningful information that you know planners can use to build seawalls," he added, "there’s going to have to be a big push to improve our projections of sea level rise using models."